Inorganic fiber papers which are essentially composed of inorganic compound fibers such as glass fibers, carbon fibers, metal fibers or ceramic fibers and processed to provide paper-like properties have recently been developed. Since various characteristic properties such as heat resistance, electric properties and mechanical properties can be provided to these fiber papers, they are used in a wide variety of fields.
For instance, the use of ceramic fiber papers composed of silica alumina fibers in filters, friction materials and adiabatic materials is now under study, making use of their excellent heat resistance, heat insulating properties and mechanical properties. Since these inorganic fibers are manufactured by melting an inorganic material as the raw material for these fibers in a furnace at 1,200 to 1,500° C. and blowing the molten product with compressed air or a spinner, they contain about 50% of non-fibrous particles. Therefore, these particles must be removed to use these fibers in inorganic fiber papers. As the fibers constituting the inorganic fiber papers which have been obtained heretofore have a relatively large diameter, it has been difficult to make the inorganic fiber papers thin. Therefore, it has been proposed that chopped strands having a fiber diameter of not more than 5 μm should be used as the main fibers in an amount of not less than 50% of the total weight of all the fibers to manufacture a glass fiber nonwoven fabric for printed circuit boards by a wet paper making process (refer to JP-A 11-189957) or that a nonwoven fabric containing not less than 50 mass % of glass chopped strands having an average fiber diameter of not less than 7 μm and 3 to 20 mass % of silica fine fibers having an average fiber diameter of not more than 1 μm should be manufactured (refer to JP-A 2004-183178). Since the fiber nonwoven fabrics obtained by these means contain glass chopped strands, they are unsatisfactory in terms of heat resistance.
Meanwhile, JP-A 2004-190183 and JP-A 2000-109306 disclose that boron nitride fibers typified by boron nitride nanotubes can be prepared by CVD. It can be expected that a compound having improved mechanical properties, heat resistance and heat conductivity can be obtained by adding these boron nitride fibers to a structural material matrix such as a polymer as an inorganic filler having high strength, high elastic modulus, high heat conductivity, insulating properties and high heat resistance. Further, it is expected that if boron nitride fibers can be molded into a thin film, they can be used as a material having higher resistance, strength and heat conductivity than conventional inorganic fibers. However, it has not been reported that a fiber paper is made from boron nitride fibers.